Field of the Invention
The present invention relates to a power transmission apparatus for a vehicle. More particularly, the present invention relates to a power transmission apparatus for a vehicle that achieves eight fixed speed stages, electric vehicle (EV) mode and hybrid electric vehicle (HEV) mode by adding a motor/generator and a planetary gear set to a double clutch transmission achieving four fixed speed stages using two synchronizers.
Description of Related Art
Environmentally-friendly techniques of vehicles are very important techniques on which survival of future motor industry is dependent. Vehicle makers are focusing on development of environmentally-friendly vehicles so as to meet environment and fuel consumption regulations.
Some examples of future vehicle techniques are an electric vehicle (EV) and a hybrid electric vehicle (HEV) that use electrical energy, and double clutch transmission (DCT) that improves efficiency and convenience.
In addition, the vehicle makers promote improvement of efficiency in a power delivery system so as to meet exhaust regulation of countries and improve fuel consumption performance. In order to improve efficiency of the power delivery system, the vehicle makers are trying to put an idle stop and go (ISG) system and a regenerative braking system to practical use.
The ISG system stops an engine when a vehicle stops and restarts the engine when the vehicle begins to run. The regenerative braking system operates a generator using kinetic energy of the vehicle instead of braking the vehicle by friction when the vehicle brakes, stores electrical energy generated at this time in a battery, and reuses the electrical energy when the vehicle runs.
In addition, the hybrid electric vehicle is a vehicle using more than two power sources, and more than two power sources are combined in various ways. Typically, the hybrid electric vehicle uses a gasoline engine or a diesel engine driven by fossil fuel and a motor/generator driven by electrical energy.
In addition, one example of a transmission applied to the hybrid electric vehicle is the DCT. According to the DCT, two clutches are applied to a manual transmission layout. Therefore, efficiency and convenience may be improved.
That is, the DCT achieves odd-numbered speed stages and even-numbered speed stages alternately by using two clutches. A mechanism achieving the odd-numbered-speeds and the even-numbered-speeds alternately improves shift feel so as to solve problems of a conventional manual transmission (MT) and an automated manual transmission (AMT).
However, the DCT has such problems that clutch damage and energy loss due to clutch slip may occur when starting, safety may not be secured since backward rolling due to clutch slip occurs excessively in hill-start, shift shock may be strong compared with an automatic transmission since shift time is controlled to be short due to thermal capacity of a clutch.
Recently, a motor/generator and a planetary gear set are added to the DCT. But, if the motor/generator and the planetary gear set are added to the DCT achieving more than six fixed speed stages, a volume of the transmission may be large and fuel economy may be deteriorated due to heavy weight.
The information disclosed in this Background of the Invention section is only for enhancement of understanding of the general background of the invention and should not be taken as an acknowledgement or any form of suggestion that this information forms the prior art already known to a person skilled in the art.